Semiconductor device and method of manufacturing the same

ABSTRACT

The sizes of crystal masses are made to be a uniform in a crystalline silicon film obtained by a thermal crystallization method in which a metal element is used. An amorphous silicon film to be crystallized is doped with a metal element that accelerates crystallization, and then irradiated with laser light (with an energy which is not large enough to melt the film and which is large enough to allow the metal element to diffuse in the solid silicon film) from the back side of a light-transmissive substrate. Thereafter, heat treatment is performed to obtain a crystalline silicon film. Thus crystal masses in the crystalline silicon film can have a uniform size and the problem of fluctuation between TFTs can be solved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor device which hasa circuit composed of a thin film transistor (hereinafter referred to asTFT), and to a method of manufacturing the semiconductor device. Forinstance, the present invention relates to an electro-optical devicerepresented by a liquid crystal display panel. and to electronicequipment which has such electro-optical device as its component.

[0003] In this specification, the term semiconductor device refers to adevice in general that utilizes semiconductor characteristics tofunction, and electro-optical devices, light emitting devices,semiconductor circuits, and electronic equipment are all semiconductordevices.

[0004] 2. Description of the Related Art

[0005] In recent years, development of a semiconductor device has beenadvanced, which has a large area integrated circuit composed of a thinfilm transistor (TFT) that is formed from a semiconductor thin film(several nm to several hundreds nm in thickness) on a substrate with aninsulating surface. Typical examples of the semiconductor device includea liquid crystal display device with a liquid crystal module, an ELmodule, and a non-magnification image sensor.

[0006] Of liquid crystal display devices, one that is attractingattention is an active matrix liquid crystal display device in whichpixel electrodes are arranged so as to form a matrix and TFTs are usedfor switching elements that are respectively connected to the pixelelectrodes in order to obtain a high quality image.

[0007] In a liquid crystal module to be mounted to a liquid crystaldisplay device, a pixel portion for displaying an image and a drivingcircuit for controlling the pixel portion are formed in differentfunctional blocks on the same substrate. The driving circuit is based ona CMOS circuit and includes a shift register circuit, a level shiftercircuit. a buffer circuit, a sampling circuit, and the like.

[0008] The pixel portion of the liquid crystal module has several tenthousands to several millions of pixels and each of the pixels isprovided with a TFT (pixel TFT). A pixel electrode is provided for eachpixel TFT. An opposite electrode is formed on an opposite substrate thatfaces the substrate which has the pixel portion and driving circuitacross the liquid crystal. A kind of capacitor is formed with the liquidcrystal as dielectric. The voltage applied to each pixel is controlledby switching function of the TFT, and electric charges given to thecapacitor are controlled to drive the liquid crystal and control theamount of light transmitted for displaying an image.

[0009] Conventionally, a TFT is formed from an amorphous silicon film.In order to achieve higher performance, it has been attempted to use acrystalline silicon film (typically called a polysilicon film) for anactive layer of a TFT (hereinafter referred to as polysilicon TFT). Apolysilicon TFT is high in field effect mobility and therefore can makea circuit which has various functions.

[0010] A technique for employing such crystalline silicon film over aglass substrate is disclosed in Japanese Patent Application Laid-openNo. Hei 8-78329. According to the technique described in thepublication, an amorphous silicon film is selectively doped with a metalelement that accelerates crystallization (typically nickel) and thensubjected to heat treatment to grow a crystalline silicon film from thedoped region. Crystal grains obtained by this technique are very largein size.

[0011] Owing to the effect of the metal element, the technique iscapable of lowering the temperature, at which the amorphous silicon filmis crystallized, by 50 to 100° C. as compared to the case where themetal element is not used in crystallization. In addition, time requiredto complete crystallization is ⅕ to {fraction (1/10)} of the case wherethe metal element is not used for crystallization. This technique issuperior also in terms of productivity.

[0012] The crystalline silicon film obtained in accordance with thetechnique of the above publication (Japanese Patent ApplicationLaid-open No. Hei 8-78329)) has a unique crystal structure. A largenumber of columnar crystal masses (also called domains) are formed inthe crystal silicon film and crystals in one crystal mass (domain) allhave the same crystal orientation. The size of one crystal mass (domain)is as large as 200 to 300 μm. Adjacent crystal masses (domains) havedifferent orientations and there is a boundary between adjacent crystalmasses. When a TFT is formed so that its channel formation region isconfined within one crystal mass, it is expected that almost the samelevel of electric characteristics as those of single crystal areobtained. However, with the technique of the above publication, crystalmasses are formed at random and it is difficult to manufacture a TFTwith its channel formation region placed within one of therandomly-formed crystal masses. Accordingly it is also difficult to forma channel formation region of every TFT in the pixel portion within onecrystal mass.

[0013] When a crystalline silicon film obtained by the technique of theabove publication is used for an active layer of a TFT, the TFT isadvantageous in excellent electric characteristics. On the other hand,there is a slight difference, namely, fluctuation., in TFTcharacteristics between TFTs obtained by the technique due to thepresence or absence of a boundary between adjacent crystal masses(between crystal masses with different orientations), or due to varyingsizes of crystal masses formed.

[0014] If there is a fluctuation in electric characteristics among theTFTs placed in the pixel portion, voltages applied to the respectivepixel electrodes are fluctuated and the amount of light transmitted isthen fluctuated, to result in uneven display to the eves of an observer.Although the fluctuation is in an acceptable range and does not cause aproblem at present, it is conceivable that the fluctuation is a veryserious problem as the pixel size is more minute to obtain an image withhigher definition in future.

[0015] With the reduction in size of a channel formation region (channellength and channel width) as wall as the upcoming reduction in the widthof a wiring line, it is unavoidable that a TFT has its channel formationregion at a boundary between crystal masses. TFT characteristics (themobility, S value, ON current value, and OFF current value, and thelike) of the TFT is different from TFT characteristics of TFTs whosechannel formation regions are not formed in boundaries, which isconsidered as a cause of display fluctuation.

[0016] Despite several attempts, there have not been found the optimummeasure to form a crystalline silicon film which has a uniform grainsize at a process temperature equal to or lower than the distortionpoint of glass substrate, namely, 60( )° C.

[0017] It is difficult to obtain a highly uniform crystalline siliconfilm and high mobility at the same time in prior art. It is alsodifficult to manufacture a TFT at a process temperature equal to orlower than 600° C. in prior art.

[0018] An active matrix type light emitting device which has an OLED asa self-luminous element (hereinafter simply referred to as lightemitting device) is being researched actively. A light emitting deviceis also called as an organic EL display (OELD) or an organic lightemitting diode (OLED).

[0019] An OLED, which is self-luminous, does not need back light whichis necessary in liquid crystal display devices (LCDs) and is thereforesuitable for making a thinner device. In addition, a self-luminous OLEDhas high visibility and no limitation in terms of viewing angle. Theseare the reasons why light emitting devices using OLEDs are attractingattention as display devices that replace CRTs and LCDs.

[0020] Known as one mode of light emitting devices using OLEDs is anactive matrix driving method in which each of pixels has a plurality ofTFTs and video signals are sequentially written in the pixels to displayan image. In the light emitting device using OLEDs, a TFT is anindispensable element when the active matrix driving method is employed.However, TFTs formed from polysilicon easily fluctuate incharacteristics due to defects in grain boundaries.

[0021] In light emitting devices using OLEDs, each pixel is providedwith at least a TFT for functioning as a switching element and a TFT forsupplying current to an OLED. Irrespective of circuit structures anddriving methods of pixels, the luminance of a pixel is determined by theON current (I_(on)) of a TFT electrically connected to an OLED to supplya current to the OLED. Therefore, when the entire screen displays white,for example, the luminance is fluctuated unless the ON current of eachpixel is constant.

SUMMARY OF THE INVENTION

[0022] In order to solve the above problems, numerous experiments andexaminations have been conducted in view of various angles. As a result,it has been found that crystal masses in a crystalline silicon film canhave a uniform size by doping the top face of an amorphous silicon filmto be crystallized with a metal element that acceleratescrystallization, irradiating the film with laser light (light from apulse oscillation laser with an energy density of 50 to 150 mJ/cm²) fromthe back side of a light-transmissive substrate, and then subjecting thefilm to heat treatment. Thus, the above problems, especially thefluctuation between TFTs, are solved to complete the present invention.

[0023] Laser light in the present invention is characterized by havingan energy density which does not melt an amorphous silicon film and bybeing capable of increasing crystal nuclei in number. There is no changein appearance before and after irradiating the laser light and no ridgesare formed on the surface of the silicon film after irradiating thelaser light. In short, the energy of laser light in the presentinvention is not large enough to melt an amorphous silicon film, is notlarge enough to change the surface state of the film, and is largeenough to allow the metal element to move in the solid semiconductorfilm.

[0024] An object of the present invention is to provide measures forobtaining a TFT with its channel formation region which has a pluralityof uniform crystal masses in size. The present invention can provide asemiconductor device which has very little fluctuation in TFTcharacteristics, and can prevent the fluctuation from being a problemwhen display with higher definition is demanded in future.

[0025] Furthermore, the present invention is capable of controlling thegrain size by adjusting the energy density of laser light before theheat treatment. Accordingly, the size of a crystal mass can beappropriately adjusted to suit the pixel size or the size of a channelformation region. Thus, the present invention prevents the fluctuationfrom being a problem when the channel formation region becomes moreminute in size in future.

[0026] Moreover, time required to complete crystallization can beshortened by the measures of the present invention if the temperature ofthe heat treatment is the same as that in the technique described in thepublication above.

[0027] According to the measures of the present invention,crystallization can be achieved with a smaller dose of metal elementthan that in the technique described in the above publication even whenthe temperature and time of the heat treatment are the same as those inthe technique described in the publication above. The metal element foraccelerating crystallization is desirably removed by a getteringtechnique or the like after crystallization and thereforecrystallization with a smaller dose of metal element is better. As thethickness of an amorphous silicon film to be crystallized is increased,the time of the heat treatment has to be prolonged, the temperature ofthe heat treatment has to be set higher. or a larger dose of metalelement is required for sufficient crystallization in prior art. On theother hand, the present invention can achieve crystallization withoutchanging heat treatment conditions even when the thickness of anamorphous semiconductor film to be crystallized is increased. In short,the present invention can crystallize a larger area of amorphoussemiconductor film in a shorter time than prior art.

[0028] It is possible to irradiate the laser light selectively beforethe heat treatment. Therefore, only a portion in which fluctuation inTFT characteristics matters, the pixel portion, for example, isirradiated with laser light while the driving circuit portion is notirradiated with laser light. Alternatively, the pixel portion and a partof the driving circuit portion may be irradiated with laser light sincethe driving circuit portion also has a portion in which fluctuation inTFT characteristics matters, as an analog switch circuit. for example.The present invention is particularly effective in a circuit which has achannel formation region with a large size, and it is possible to placetwo or more crystal masses (domains) in the channel formation region inthe channel length direction.

[0029] The present invention disclosed in this specification relates toa method of manufacturing a semiconductor device, including: a firststep of forming a semiconductor film with an amorphous structure on asubstrate with an insulating surface: a second step of adding a metalelement to the semiconductor film with an amorphous structure a thirdstep of irradiating light to the semiconductor film with an amorphousstructure from the back side of the substrate through the substrate: anda fourth step of forming a semiconductor film with a crystal structureby heating the semiconductor film with an amorphous structure.

[0030] The present invention also relates to another method ofmanufacturing a semiconductor device, including: a first step of forminga semiconductor film with an amorphous structure on a substrate with anuneven insulating surface; a second step of adding a metal element tothe semiconductor film with an amorphous structure; a third step ofirradiating light to the semiconductor film with an amorphous structurefrom the back side of the substrate through the substrate; and a fourthstep of forming a semiconductor film with a crystal structure by heatingthe semiconductor film with an amorphous structure.

[0031] The uneven insulating surface in the above refers to a surface ofan insulating film (such as a gate insulating film or an interlayerinsulating film) formed so as to cover an electrode (such as a gateelectrode) on a substrate, or a surface of a base insulating film formedon a substrate, or a surface of an insulating substrate.

[0032] The above manufacturing methods are characterized in that crystalmasses in the semiconductor film with a crystal structure aresubstantially uniform in size, and are also characterized in that sizesof crystal masses in the semiconductor film with a crystal structure areapproximately 1 to 20 μm.

[0033] The above manufacturing methods are characterized in that thelight is pulse oscillation laser light which has an energy density of 50to 150 mJ/cm². The present invention can employ a continuous wave laserother than a pulse oscillation laser. The light is emitted from one ormore kinds of lasers selected from the group consisting of a pulseoscillation excimer laser, a pulse oscillation Kr laser, a pulseoscillation Kr laser, a continuous wave excimer laser, a continuous waveAr laser, and a continuous wave Kr laser.

[0034] Further, it is needless to say that it is possible to use a lightemitted from one or more kinds of lasers selected from the groupconsisting of a Continuous wave YAG laser, a continuous wave YVO₄ laser,a continuous wave YLF laser, a continuous wave YAlO₃ laser, a continuouswave glass laser, a continuous wave ruby laser, a continuous wavealexandrite laser, a continuous wave Ti : sapphire laser, a pulseoscillation YAG laser, a pulse oscillation YVO₄ laser, a pulseoscillation YLF laser, a pulse oscillation YAlO₃ laser, a pulseoscillation glass laser, a pulse oscillation ruby laser, a pulseoscillation alexandrite laser, and a pulse oscillation Ti : sapphirelaser.

[0035] When a continuous wave laser is used, it is important for thelaser light to have an energy density which does not melt thesemiconductor film. When the energy density is set to 0.01 to 100 MW/cm²(preferably 0.1 to 10 MW/cm²) and the scanning rate is set to 0.5 to2000 cm/sec., the metal element is uniformly diffused throughout thesemiconductor film to increase crystal nuclei in number while thesemiconductor film is kept in its solid.

[0036] The above manufacturing methods are characterized in that themetal element is one that accelerates crystallization. In thisspecification, a metal element that accelerates crystallization is oneor more kinds of elements selected from the group consisting of Fe, Ni,Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.

[0037] Also, the laser light may be irradiated before crystallization atthe energy which is not large enough to melt the semiconductor film andnot large enough to change the surface state. The present inventionrelates to another method of manufacturing a semiconductor device,including: a first step of forming a semiconductor film with anamorphous structure on a substrate with an insulating surface; a secondstep of irradiating the front side or back side of the semiconductorfilm with an amorphous structure by using light with an energy which isnot large enough to melt the semiconductor film; and a third step offorming a semiconductor film with a crystal structure by crystallizingthe semiconductor film with an amorphous structure.

[0038] In the above method of manufacturing a semiconductor device, thethird step may be a step of irradiating laser light (light with anenergy which is large enough to melt the semiconductor film) to thesemiconductor film with an amorphous structure, or a step of heating thesemiconductor film with an amorphous structure, or a step of heating thesemiconductor film with an amorphous structure after doping theamorphous semiconductor film with a metal element that acceleratescrystallization.

[0039] As has been described, it makes an impurity contained in thesemiconductor film (an element which has a high diffusion constant or ahigh degree of solid solution in the semiconductor film, hydrogen, forexample) diffuse in the solid film for performing improvedcrystallization later to irradiate laser light with an energy which isnot large enough to melt a semiconductor film with an amorphousstructure before crystallization. When a semiconductor film with auniform amorphous structure, obtained by irradiating laser light with anenergy which is not large enough to melt the semiconductor film, iscrystallized, a semiconductor film with a uniform crystal structure canbe obtained. Accordingly, TFTs using as their active layers thissemiconductor film with a crystal structure are uniform incharacteristics to reduce uneven display and fluctuation in luminance.

[0040] Also, irradiation of laser light may selectively be performed.The present invention relates to another method of manufacturing asemiconductor device which has a pixel portion and a driving circuit onthe same substrate, including: a first step of forming a semiconductorfilm with an amorphous structure on a substrate with an insulatingsurface; a second step of selectively irradiating only a region of thesemiconductor film with an amorphous structure that serves as the pixelportion with light with an energy which is not large enough to melt thesemiconductor film: and a third step of forming a semiconductor filmwith a crystal structure by heating the semiconductor film with anamorphous structure.

[0041] The semiconductor device obtained by the above manufacturingmethods has characteristics uniquely provided by the present invention.In the present invention, there is provided a semiconductor device whichhas a first region comprising a first TFT and a second region comprisinga second TFT on the same substrate, in which the first TFT uses as itsactive layer a semiconductor film with a crystal structure, sizes ofcrystal grains are larger in the first TFT than in the second TFT, andsizes of crystal grains are less fluctuated in the first region than inthe second region.

[0042] Selective irradiation of laser light (with energy which is notlarge enough to melt a semiconductor film) makes it possible to formdifferent TFTs so as to suit different circuits. For instance, regionsfor forming TFTs used for a circuit in which the fluctuation matters,such as a pixel TFT used as a switching element and a TFT of an analogswitch circuit, are irradiated with laser light (with energy which isnot large enough to melt a semiconductor film) whereas regions forforming TFTs used for a circuit in which an increase in ON currentmatters more than reduced fluctuation, such as a TFT of a buffercircuit, are not irradiated with the laser light. Then, crystallizationis performed through heat treatment to selectively form regions wherecrystal grains are small and the fluctuation is small (regionsirradiated with the laser light) and regions where crystal grains arelarge (regions that are not irradiated with light).

[0043] The above structure is characterized in that the first region isa pixel portion and the second region is a driving circuit. If a displayunit has a pixel TFT in which there is the fluctuation in size is smallbetween crystal grains, it is possible to complete a display device(typically a liquid crystal display device or a display device with anOLED) which is free from uneven display using the display unit.

[0044] In this specification, an electrode is a part of a wiring lineand refers to a point at which one wiring line is electrically connectedto another wiring line, or a point at which a wiring line intersects asemiconductor layer. Although he terms -“wiring line” and “electrode”are distinguished from each other for the sake of explanation“electrode” always implies “wiring line”.

[0045] An organic light emitting layer is defined in this specificationas an aggregate of layers formed between an anode and cathode of anOLED. Specifically, an organic light emitting layer includes a lightemitting layer, a hole injecting layer, an electron injecting layer, ahole transporting layer, an electron transporting layer, etc. The basicstructure of OLED is a laminate of an anode, a light emitting layer, anda cathode layered in order. The basic structure may be modified into alaminate of an anode, a hole injecting layer, a light emitting layer,and a cathode layered in order, or a laminate of an anode, a holeinjecting layer, a light emitting layer, an electron transporting layer,and a cathode layered in order.

[0046] An OLED has, in addition to an anode and a cathode, a layercontaining an organic compound (organic light emitting material) thatgenerates luminescence (electro luminescence) when an electric field isapplied (the layer is hereinafter referred to as organic light emittinglayer). Luminescence obtained from organic compounds is classified intolight emission in returning to the base state from singlet excitation(fluorescence) and light emission in returning to the base state fromtriplet excitation (phosphorescence). A light emitting device of thepresent invention may use one or both of the above two types of lightemission.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In the accompanying drawings:

[0048]FIG. 1 is a graph showing results of TXRF performed on a siliconsurface after laser light irradiation;

[0049]FIG. 2 is a graph showing the relation between the energy densityof laser light irradiated from the back side and the size of crystalmass;

[0050]FIG. 3 is a picture observing a surface of a silicon film of thepresent invention (magnification: ×100);

[0051]FIGS. 4A to 4G are diagrams showing a manufacturing process of thepresent invention (Embodiment 1);

[0052]FIGS. 5A to 5E are diagrams showing a manufacturing process of thepresent invention (Embodiment 2);

[0053]FIGS. 6A to 6D are diagrams showing a process of manufacturing anAM-LCD (Embodiment 3);

[0054]FIGS. 7A to 7D are diagrams which show a process of manufacturingan AM-LCD;

[0055]FIGS. 8A to 8D are diagrams which show a process of manufacturingan AM-LCD;

[0056]FIGS. 9A to 9C are diagrams which show a process of manufacturingan AM-LCD;

[0057]FIGS. 10A and 10B are diagrams which show a process ofmanufacturing an AM-LCD;

[0058]FIGS. 11A and 11B are graphs showing the OFF current value ofn-channel TFTs (L/W=8/8);

[0059]FIG. 12 is a graph showing the field effect mobility of n-channelTFTs (L/W=8/8);

[0060]FIGS. 13A and 13B are graphs showing the OFF current value ofn-channel TFTs (L/W=50/50);

[0061]FIG. 14 is a graph showing the field effect mobility of n-channelTFTs (L/W=50/50);

[0062]FIG. 15 is a picture observing a surface of a silicon film of acomparative sample (magnification: ×100);

[0063]FIGS. 16A and 16B are diagrams showing a manufacturing process ofthe present invention (Embodiment 4);

[0064]FIG. 17 is a diagram showing a liquid crystal module (Embodiment5);

[0065]FIGS. 18A to 18F are diagrams showing electronic equipment(Embodiment 7);

[0066]FIGS. 19A to 19D are diagrams showing electronic equipment(Embodiment 7); and

[0067]FIGS. 20A to 20C are diagrams showing electronic equipment(Embodiment 7).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] [Embodiment Mode]

[0069] An embodiment mode of the present invention will be describedbelow.

[0070] The present invention includes a first step of doping a surfaceof an amorphous semiconductor film with a metal element, a second stepof irradiating laser light (an energy density of 50 to 150 mJ/cm²) fromthe back side, and a third step of performing a heat treatment forcrystallization. Through the three steps, the present invention obtainsa crystalline semiconductor film which has uniform crystal masses andcontrols sizes of the crystal masses in the crystalline semiconductorfilm.

[0071] The amorphous semiconductor film can be formed by reducedpressure thermal CVD, plasma CVD, sputtering, or the like from asemiconductor material, for example, silicon or a silicon germanium(Si_(X)Ge_(I−X) (X=0.0001 to 0.02) alloy.

[0072] The doped metal element before crystallization in order toaccelerate crystallization is desirably removed from or reduced in thecrystalline semiconductor film by gettering after the crystallization.Doping of the metal element that accelerates crystallization can beachieved by application of a solution containing the metal element, orby forming a thin film through sputtering or CVD. One employablegettering method includes the steps of forming an oxide film on thecrystalline silicon film; forming an amorphous silicon film containingnoble gas (typically argon) as a gettering site on the oxide film toform a laminate; and performing a heat treatment to move the metalelement (typically nickel) in the crystalline silicon film to thegettering site and remove the metal element from the crystalline siliconfilm, or reduce the concentration of the metal element in thecrystalline silicon film. Another employable gettering method includesthe steps of doping a part of the crystalline silicon film withphosphorus or noble gas to form a gettering site; and perform a heattreatment to move the metal element (typically nickel) from the getteredregion to the gettering site.

[0073] The laser light is obtained from gas laser or solid laser.Examples of gas laser include an excimer laser, an Ar laser, and a Krlaser. Examples of solid laser include a YAG laser, a YVO₄ laser, a YLFlaser, a YAlO₃ laser, a glass laser, a ruby laser, an alexandrite laser,and a Ti : sapphire laser. A solid laser employed uses a crystal of YAG,YVO₄, YLF, or YAlO₃ doped with Cr, Nd, Er, Ho, Ce, Co, Ti, or Tm. Thefundamental wave of light emitted from the laser varies in accordancewith the dopant and has a wavelength of about 1 μm. Harmonic is obtainedfrom the fundamental wave by using a non-linear optical element.Particularly preferred are the second harmonic or third harmonic of apulse oscillation excimer laser, continuous wave excimer laser, thesecond harmonic or the third harmonic of continuous wave YAG laser, orthe second harmonic of a YVO₄ laser. The shape of the laser light to beirradiated may be linear, rectangular, or elliptical.

[0074] The description below deals with a case where pulse oscillationlaser light (an energy density of 50 to 150 mJ/cm²) is employed.However, it is needless to say that continuous wave laser light can beused instead.

[0075] According to the present invention, a crystalline semiconductorfilm in which crystal masses have the uniform size can be obtained, achannel formation region can have a plurality of crystal masses (aplurality of domains) wherever it is positioned. and a boundary betweencrystal masses is scattered evenly in the channel formation region.Therefore, the present invention can provide uniform electriccharacteristics.

[0076] The present invention is also capable of shortening time requiredfor crystallization since a large number of crystal nuclei are formed byirradiation of laser light (an energy density of 50 to 150 mJ/cm²) fromthe back side and then heat treatment follows to further increase thenumber of crystal nuclei and complete crystallization. In addition, thepresent invention can achieve crystallization in a short period of timeeven when a temperature in the heat treatment is lower than in priorart. Furthermore, the present invention can crystallize a thicksemiconductor film, for example, an amorphous silicon film with athickness of 80 nm or more, without increasing the dose of the metalelement that accelerates crystallization.

[0077] An observation through a microscope after irradiating the laserlight from the back side finds no particular changes compared the statebefore irradiating the laser light. The laser light (an energy densityof 50 to 150 mJ/cm²) is very weak as compared to the intensity of laserlight used in crystallization (an energy density of 200 to 250 mJ/cm²,or more). At the energy density of the laser light in the presentinvention, the semiconductor film is not melted and no ridges areformed. When irradiation of this weak laser light with a small energydensity is followed by heat treatment for crystallization. a largenumber of crystal masses with a uniform size can be formed.

[0078] To unravel the mechanism of forming crystal masses with uniformsize. the following experiments have been conducted.

[0079] Experiment 1

[0080] First, a silicon oxide film with a thickness of 280 nm and anamorphous silicon film with a thickness of 69 nm are formed by reducedpressure thermal CVD and layered on a quartz substrate and a nickelacetate aqueous solution (5 ppm) is applied thereto to prepare a sample.While laser light conditions are varied (including the energy densityand the number of shots), samples are irradiated with laser light. Then,the nickel detection intensity and the silicon detection intensity onthe surfaces are measured by TXRF (total reflection X-ray fluorescence)to calculate the ratio of the nickel detection intensity to the silicondetection intensity. Experiment 1 is an experiment for examining thepresence or absence of a change in nickel concentration immediatelyafter irradiating the laser light.

[0081]FIG. 1 shows the results of Experiment 1. In FIG. 1, samplesirradiated with excimer laser light (energy density: 60 mJ/cm², 90mJ/cm², 120 mJ/cm², 150 mJ/cm², 180 mJ/cm²) of 132 shots from the backside of the substrate are plotted and indicated by □. Samples irradiatedwith excimer laser light (energy density: 60 mJ/cm², 90 mJ/cm², 120mJ/cm², 150 mJ/cm², 180 mJ/cm²) of 13 shots from the back side of thesubstrate are plotted and indicated by ◯. For comparison, samplesirradiated with excimer laser light (energy density: 60 mJ/cm², 90mJ/cm², 120 mJ/cm², 150 mJ/cm², 180 mJ/cm²) of 13 shots from the frontside of the substrate are plotted and indicated by ▪, and samplesirradiated with excimer laser light (energy density: 60 mJ/cm², 90mJ/cm², 120 mJ/cm², 150 mJ/cm², 180 mJ/cm²) of 13 shots from the frontside of the substrate are plotted and indicated by . Other comparativesamples that are not irradiated with laser light are plotted andindicated by ▴.

[0082] Experiment 2

[0083] The samples irradiated with laser light from the back side inExperiment 1 are subjected to heat treatment (heat treatment at 450° C.for an hour which is followed by heat treatment at 600° C. for twelvehours) for crystallization. The samples are then observed through anoptical microscope to measure sizes of crystal masses. Experiment 2 isan experiment for examining the relation between the size of the crystalmass and the energy density in the case where irradiation of laser lightfrom the back side precedes heat treatment for crystallization.

[0084]FIG. 2 shows the results of Experiment 2. In FIG. 2, samplesirradiated with excimer laser light (energy density: 60 mJ/cm², 90mJ/cm², 120 mJ/cm², 120 mJ/cm², 180 mJ/cM²) of 132 shots from the backside of the substrate are plotted and indicated by □. Samples irradiatedwith excimer laser light (energy density: 60 mJ/cm², 90 mJ/cm², 120mJ/cm², 150 mJ/cm², 180 mJ/cm²) of 13 shots from the back side of thesubstrate are plotted and indicated by ◯. Samples irradiated withexcimer laser light (energy density: 60 mJ/cm², 90 mJ/cm², 120 mJ/cm²,150 mJ/cm², 180 mJ/cm²) of 66 shots from the back side of the substrateare plotted and indicated by ▴.

[0085] From these experiment results, the present inventors haveconcluded that the mechanism for forming crystal masses with an uniformsize is as follows:

[0086] In the laser irradiation, photo reaction brought by the laserlight severs Si-Si lattice bond, which is substituted for a metal(nickel) to accelerate silicide (NiSi_(x)) reaction. In other words,nickel is diffused in the film by the laser irradiation to formsilicides (NiSi_(x)). FIG. 1 shows that nickel on the surface is reducedin amount as laser light with an energy density of 60 mJ/cm² or moremakes nickel diffuse in the film. That is. the laser irradiation makesnickel move and diffuse in the solid silicon film. More silicides(NiSi_(x)) are formed in the subsequent heat treatment. The thus formedsilicides serve as crystal nuclei to advance crystallization. As thenumber of the crystal nuclei are increased, crystal grains and crystalmasses are reduced in size. In this way, the present invention makes thesize of the crystal mass uniform by increasing the number of crystalnuclei. In FIG. 2, the smallest size of the crystal mass, about 5 μm, isobtained under the condition of light with an energy density of 90mJ/cm² in 132 shots. A picture of the sample observed through an opticalmicroscope is shown in FIG. 3.

[0087] It is another method for increasing the number of crystal nucleito dope the semiconductor film with a large amount of metal element thataccelerates crystallization, and the surface energy and chemicalpotential of the semiconductor film are changed. Although this methoddoes increase the number of crystal nuclei, it has a problem of themetal element remaining as metal compounds in an excessive amount in ahigh resistance region (such as a channel formation region and an offsetregion). It is easy that a current flows in the metal compounds, andtherefore the metal compounds lower the resistance of the region that issupposed to be a high resistance region and impair stability of TFTelectric characteristics as well as reliability.

[0088] In the case where the semiconductor film is crystallized byperforming only heat treatment after doping a metal element withoutlaser irradiation from the back side, the number of crystal masses issmall and sizes of crystal masses range between 200 μm and 300 μm. Thesample crystallized by performing only heat treatment is observedthrough an optical microscope and a picture thereof is shown in FIG. 15.

[0089] In the case where the semiconductor film is crystallized byperforming heat treatment after laser irradiation under the sameconditions except irradiating laser light from the front side, crystalmasses are smaller in size than the case where the semiconductor film iscrystallized by performing only heat treatment. However, the size of thecrystal mass in this case is larger than 5 μm and widely varies unlikethe case where the semiconductor film is irradiated with laser lightfrom the back side. Since the energy density received by thesemiconductor film itself in the case of irradiating from the back sideis different from that in the case of irradiating from the front side,the energy density is converted to set the effective energy density inthe case of irradiating from the back side equal to the effective energydensity in the case of irradiating from the front side. Still, the sizeof the crystal mass in the case of irradiating from the front side islarger than 5 μm and widely varies unlike the case of irradiating fromthe back side. From these facts, it is inferred that the key for formingcrystal masses with uniform size is irradiation of the semiconductorfilm with laser light from the back side, not the laser energy density.

[0090] When a semiconductor film is crystallized using a metal elementthat accelerates crystallization, crystal grains grown from crystalnuclei formed due to the metal element are mixed with crystal grainsgrown from natural nuclei (natural nuclei defined in this specificationare crystal nuclei except those formed due to a metal element) to causefluctuation in physical property of the semiconductor film. It is knownthat natural nuclei are generated when the temperature is 600° C. orhigher or when it takes long for the semiconductor film to crystallize.Such fluctuation causes fluctuation in electric characteristics anduneven display when the semiconductor film is used in a display unit ofa semiconductor device.

[0091] The present invention prevents natural nuclei from beinggenerated and controls the number of crystal nuclei to obtain uniformcrystal masses and control sizes of the crystal masses. Although he sizeof the crystal mass in the present invention is 1 to 20 μm, it is notparticularly limited. The appropriate size of the crystal mass variesbetween one type of TFT from another and it should be set in accordancewith the type of TFT. In the present invention, the size of the crystalmass can be set freely by adjusting laser light the energy density andthe number of shots. In short, the present invention can control thesize of the crystal mass freely within a range between 1 μm and 20 μmand can provide a semiconductor film with uniform characteristics.Accordingly, TFTs which have as active layers semiconductor films withuniform characteristics can have uniform electric characteristics.

[0092] In the present invention, irradiation of laser light beforecrystallization does not form a ridge on the surface of thesemiconductor film since the semiconductor film is crystallized by heattreatment in a furnace. In conventional crystallization by laser light,growing crystals collide with each other and form ridges to make thesurface of the semiconductor layer uneven and lower the OFF currentvalue. Furthermore, in conventional crystallization by laser light,crystal nuclei are formed at random during a cooling period after thelaser irradiation, and crystals grow in various directions to formminute crystal grains with various grain sizes and a large number ofcrystal defects. Moreover, conventional crystallization by laser lightfails to give a uniform energy to the entire film and leaves wave-liketracks after irradiating laser light. In contrast to prior art, thepresent invention employs heat treatment for crystallization and iscapable of giving a uniform energy to the entire film.

[0093] In conventional solid phase growth by annealing at hightemperature, crystals grow from natural nuclei and the natural nucleiare generated at random. On the other hand, the present invention cancontrol the generation of crystal nuclei by doping an amorphoussemiconductor film with a metal element and then irradiating laser light(with energy which is not large enough to melt the semiconductor filmand which is large enough to allow the metal element to diffuse in thesolid semiconductor film) from the back side.

[0094] In conventional crystallization methods (laser crystallizationand solid phase growth), crystals formed in a plane have various grainsizes. The grain size may be locally small and does not exceed 1 μmthroughout the semiconductor film.

[0095] Another experiment has been conducted, in which an insulatingfilm is formed to cover a wiring line, the insulating film which has anuneven surface due to the wiring line is covered with an amorphoussemiconductor film, and the amorphous semiconductor film is doped with ametal element, irradiated with laser light from the back side. and thensubjected to heat treatment. As a result, uniform crystal grains areobtained despite the uneven surface. This experimental result shows thatcrystal nuclei generated by the metal element and by laser light fromthe back side dominate in growing crystals although unevenness can causegeneration of crystal nuclei. Therefore, according to the presentinvention, uniform crystal grains can be obtained in spite of unevennessof the substrate and can also be obtained when a wiring line is placedunder the semiconductor film.

[0096] The present invention is very effective not only in a top gateTFT but also in a bottom gate TFT. In a bottom gate TFT, an amorphoussilicon film formed on an insulating film that covers a gate electrodeis crystallized. The amorphous silicon film is crystallized to greatlydifferent degrees in a region that overlaps the gate electrode and aregion that does not overlap the gate electrode in prior art. Thepresent invention can uniformly crystallize the entire amorphous siliconfilm formed on an insulating film that covers the gate electrode.

[0097] The number of crystal nuclei may be increased even more beforecrystallization by exposing the amorphous silicon film to an atmosphereof plasmatized gas that mainly contains one or more kinds selected froma noble gas element, nitrogen, and ammonium.

[0098] A pulse oscillation excimer laser is used in the experimentsdescribed above. However, similar results can be obtained when acontinuous wave laser is used as long as the energy of laser light isproperly adjusted so as not to melt the semiconductor film.

[0099] The present invention with the above structure will further bedescribed in detail through the following embodiments.

[0100] [Embodiment 1]

[0101] Embodiment 1 describes an example of manufacturing a TFT from anamorphous semiconductor film obtained by reduced pressure thermal CVD ona quartz substrate (1.1 mm in thickness). The description is given withreference to FIGS. 4A to 4G.

[0102] First, an amorphous silicon film 101 with a thickness of 50 nmand a silicon oxide film 102 with a thickness of 50 nm are formed oneach side of a quartz substrate 100 by reduced pressure thermal CVD(FIG. 4A).

[0103] A resist film 103 is formed next in order to remove the amorphoussilicon film and silicon oxide film formed on the back side of thesubstrate. The silicon oxide film on the back side is removed by asolution containing fluoric acid, and the amorphous silicon film on theback side is removed by a mixture gas of SF₆ and He (FIG. 4A).

[0104] Next, the resist film 103 is removed and the silicon oxide film102 on the front side is removed. After washing the substrate withdiluted fluoric acid, an oxide film (not shown) is formed by using ozonewater on the surface of the amorphous silicon film. A solutioncontaining nickel (5 ppm) is applied by spin coating to form a thinmetal film 104 (FIG. 4C). Here, nickel is used as a metal element thataccelerates crystallization. Nickel is a metal element which is veryhigh in diffusion constant and degree of solid solution in a siliconfilm, and therefore is suitable for the present invention.

[0105] Then, linear laser light is irradiated from the back side of thesubstrate (FIG. 4D). An excimer laser (XeCl: 308 nm) is employed here.Conditions in irradiating the laser light include the energy density of50 to 150 mJ/cm², the frequency of 30 Hz. the number of shots of 66 to132, and the scanning rate of 0.1 mm/sec. When the frequency is sethither than 30 Hz, the scanning rate can be set exceed 0.1 mm/sec. Here,the energy sensity is set to 100 mJ/cm², the frequency to 30 Hz, thenumber of shots to 132, and the scanning rate to 0.1 mm/sec. forirradiating the laser light. Crystal nuclei are generated through thislaser light. However, the laser light is irradiated for increasing thenumber of crystal nuclei and most part of the semiconductor film remainsamorphous. The semiconductor film at this point is a semiconductor film105 with an amorphous structure in which the density of crystal nucleiis higher than in the semiconductor film 101 with an amorphousstructure. FIG. 1 shows results of measuring the surface of thesemiconductor film at this stage by TXRF. From FIG. 1, it can be readthat the laser light with an energy density which is not large enough tomelt the silicon film makes nickel on the surface to diffuse into thefilm to form silicides that serve as crystal nuclei.

[0106] Next, the semiconductor film is crystallized by heat treatment toform a semiconductor film 106 with a crystal structure (FIG. 4E). Here,heat treatment at 450° C. for an hour is followed by heat treatment at600° C. for twelve hours. The thus obtained semiconductor film 106 witha crystal structure has relatively large crystal masses in size, about 5μm, and the crystal masses are uniform throughout the film. Thesemiconductor film at this stage is subjected to etching and a pictureof the surface observed through an optical microscope is shown in FIG.3.

[0107] The size of the crystal mass can be set suitably by adjustinglaser in the energy density, the number of shots (overlap ratio), andscanning rate.

[0108] Next, the obtained semiconductor film 106 with a crystalstructure is patterned to form a semiconductor layer 107 (FIG. 4F).

[0109] After washing the surface of the semiconductor layer with anetchant containing fluoric acid, an insulating film mainly containingsilicon is formed to serve as a gate insulating film 108. The surfacewashing and formation of the gate insulating film are desirably carriedout in succession without exposing the substrate to the air.

[0110] The surface of the gate insulating film is washed and then a gateelectrode 109 is formed. The semiconductor layer is doped with animpurity element that gives a semiconductor the n-type conductivity(such as P or As), here. phosphorus. to form a source region 110 and adrain region 111. After the doping, the impurity element is activated byheat treatment, irradiation of intense light, or irradiation of laserlight. The impurity element is activated, and at the same time, plasmadamage to the gate insulating film and plasma damage to the interfacebetween the gate insulating film and the semiconductor layer can berepaired. It is particularly effective to activate the impurity elementby irradiating the second harmonic of a YAG laser from the front side orback side in an atmosphere at room temperature to 300° C. A YAG laserwhich requires little maintenance is preferable measures for activation.

[0111] When heat treatment is employed as activation measures,activation and gettering can be achieved simultaneously. Gettering hereutilizes phosphorus doped in the source region or drain region. Themetal element that accelerates crystal growth, doped beforecrystallization, is desirably removed from or reduced in the crystallinesemiconductor film through gettering after the crystallization.

[0112] The subsequent steps include forming an interlayer insulatingfilm 113, hydrogenating the semiconductor layer, forming contact holesthat reach the source region and the drain region, and forming a sourceelectrode 114 and a drain electrode 115. A TFT is thus completed (FIG.4G).

[0113] Although the thus obtained TFT has a plurality of grainboundaries in a channel formation region 112, highly uniform crystalmasses are obtained. There is little fluctuation between TFTs formed onthe substrate.

[0114] The present invention is not limited to the structure of FIGS. 4Ato 4G. If necessary, the present invention can take a lightly dopeddrain (LDD) structure in which an LDD region is formed between a channelformation region and a drain region (or a source region). In the LDDstructure, a region lightly doped with an impurity element (LDD region)is provided between a channel formation region and a source region ordrain region heavily doped with an impurity element. The presentinvention may also take a so-called GOLD (gate-drain overlapped LDD)structure in which an LDD region overlaps a gate electrode with a gateinsulating film interposed therebetween.

[0115] Although an n-channel TFT is described here, a p-channel TFT canbe formed instead if an n-type impurity element is replaced by a p-typeimpurity element.

[0116] Although a top gate TFT is taken as an example in the descriptionhere, the present invention can be applied to any TFT structure. Forinstance, the present invention is applicable to a bottom gate (reversestagger) TFT and a forward stagger TFT.

[0117] [Embodiment 2]

[0118] This embodiment describes an example of manufacturing a TFT froman amorphous semiconductor film formed by plasma CVD on a glasssubstrate. The description will be given with reference to FIGS. 5A to5E.

[0119] In FIG. 5A, reference symbol 200 denotes a substrate with aninsulating surface. 201, a base insulating film, and 202, asemiconductor film with an amorphous structure.

[0120] First, an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride film is formed on the substrate200 as the base insulating film 201 which serves as a blocking layer.The base insulating film 201 here has a two-layer structure (a siliconoxynitride film with a thickness of 50 nm and a silicon oxynitride filmwith a thickness of 100 nm). However, the base insulating film may be asingle layer or a laminate which has more than two layers. The baseinsulating film may be omitted when a blocking layer is not necessary.

[0121] Next, the semiconductor film 202 with an amorphous structure isformed on the base insulating film by plasma CVD. When a semiconductorfilm with an amorphous structure is formed by plasma CVD, it is easierto form crystal nuclei than in a semiconductor film obtained by reducedpressure thermal CVD.

[0122] The surface of the semiconductor film 202 with an amorphousstructure is irradiated with an ultraviolet ray in an atmospherecontaining oxygen to form an oxide film (not shown), and a nickel thinfilm 203 is formed on the oxide film by sputtering (FIG. 5A). Although anickel thin film is used here, nickel may be sprayed and scattered onthe surface instead of forming a film.

[0123] Next, the semiconductor film is irradiated with linear excimerlaser light from the back side of the substrate (FIG. 5B) to generatecrystal nuclei through this laser light irradiation. However, the laserlight is irradiated for increasing the number of crystal nuclei and mostpart of the semiconductor film remains amorphous. The semiconductor filmat this point is a semiconductor film 204 with an amorphous structure inwhich the density of crystal nuclei is higher than in the semiconductorfilm 202 with an amorphous structure.

[0124] Next, the semiconductor film is crystallized by heat treatment toform a semiconductor film 205 with a crystal structure (FIG. 5C). Here,heat treatment at 450° C. for an hour is followed by heat treatment at600° C. for twelve hours. The thus obtained semiconductor film 205 witha crystal structure has a relatively large grain size, about 5 μm, andthe grain size is uniform throughout the film.

[0125] The grain size can be set suitably by adjusting laser in theenergy clensity, the number of shots (overlap ratio), and the scanningrate.

[0126] Next, the obtained semiconductor film 205 with a crystalstructure is patterned to form a semiconductor layer 206 (FIG. 5D).

[0127] After washing the surface of the semiconductor layer with anetchant containing fluoric acid, an insulating film mainly containingsilicon is tormed to serve as a gate insulating film 207. The surfacewashing and formation of the gate insulating film are desirably carriedout in succession without exposing the substrate to the air.

[0128] The surface of the gate insulating film is washed and then a gateelectrode 208 is formed. The semiconductor layer is doped with animpurity element that gives a semiconductor the n-type conductivity(such as P or As), here, phosphorus, to form a source region 209 and adrain region 210. After the doping, the impurity element is activated byheat treatment, irradiation of intense light, or irradiation of laserlight. The impurity element is activated, and at the same time, plasmadamage to the gate insulating film and plasma damage to the interfacebetween the gate insulating film and the semiconductor layer can berepaired. It is particularly effective to activate the impurity elementby irradiating the second harmonic of a YAG laser from the front side orback side in an atmosphere at room temperature to 300° C. A YAG laserwhich requires little maintenance is preferable measures for activation.

[0129] When heat treatment is employed as activation measures.activation and gettering can be achieved simultaneously. Gettering hereutilizes phosphorus doped in the source region or drain region. Themetal element that accelerates crystal growth, doped beforecrystallization, is desirably removed from or reduced in the crystallinesemiconductor film through gettering after the crystallization.

[0130] The subsequent steps include forming an interlayer insulatingfilm 212, hydrogenating the semiconductor layer. forming contact holesthat reach the source region and the drain region, and forming a sourceelectrode 213 and a drain electrode 214. A TFT is thus completed (FIG.5E).

[0131] Although the thus obtained TFT has a plurality of grainboundaries in a channel formation region 211, highly uniform crystalmasses are obtained. There is little fluctuation between TFTs formed onthe substrate.

[0132] [Embodiment 3]

[0133] Described here is a method of manufacturing a liquid crystaldisplay device using an active matrix substrate which has a pixelportion. The description will be given with reference to FIGS. 6A to10B.

[0134] An active matrix liquid crystal display device that uses a TFT asa switching element has a substrate on which pixel electrodes arearranged so as to form a matrix (active matrix substrate) and anopposite substrate on which an opposite electrode is formed. The activematrix substrate and the opposite substrate face each other via a liquidcrystal layer. The distance between the substrates is kept constant by aspacer or the like. A liquid crystal layer is sealed between thesubstrates by a seal member placed on the outer periphery of the pixelportion.

[0135] An example of manufacturing active matrix substrate is givenbelow.

[0136] First, a conductive film is formed on a substrate 401 which hasan insulating surface and is patterned to form a scanning line 402 (FIG.6A). The scanning line 402 also functions as a light-shielding layer forprotecting an active layer to be formed later from light. Here. a quartzsubstrate is used for the substrate 401 and the scanning line 402 is alaminate of a polysilicon film (75 nm in thickness) and a tungstensilicide (W-Si) film (150 nm in thickness). The polysilicon film alsoprotects the substrate from contamination by tungsten silicide.

[0137] Next, insulating films 403 a and 403 b are formed to have athickness of 100 to 1000 nm (typically 300 to 600 nm) to cover thescanning line 402 (FIG. 6B). Here, a silicon oxide film formed by CVD tohave a thickness of 100 nm and a silicon oxide film formed by LPCVD tohave a thickness of 480 nm are layered.

[0138] After the insulating film 403 b is formed, the surface of theinsulating film may be leveled by chemical-mechanical polishing calledCMP. When CMP is employed, a suitable polishing agent (slurry) for aninsulating film is, for example, fumed silica particles dispersed in aKOH-doped aqueous solution. Fumed silica is obtained by thermal crackingof silicon chloride gas. The insulating film is thinned by 0.1 to 0.5 μmwith CMP to level the surface. For example, the insulating film ispolished so that the insulating film is 0.5 μm in height. preferably,0.3 μm, at the highest point (Rmax). After CMP, the surface of theinsulating film is desirably washed with an etchant containing fluoricacid in order to remove impurities such as K (potassium).

[0139] Next, an amorphous semiconductor film is formed to have athickness of 10 to 100 nm. Here, reduced pressure thermal CVD is used toform an amorphous silicon film with a thickness of 50 nm. A siliconoxide film with a thickness of 50 nm is formed on the amorphous siliconfilm by reduced pressure thermal CVD. In reduced pressure thermal CVD, afilm is formed on each side of a substrate. Therefore, after a resistfilm is formed on the front side of the substrate, the silicon oxidefilm on the back side is removed by a solution containing fluoric acidand the amorphous silicon film on the back side is removed by a mixturegas of SF₆ and He. After the films on the back side are removed, theresist film is removed to remove the silicon oxide film on the frontside.

[0140] The amorphous semiconductor film is then crystallized. InEmbodiment 3, the entire surface of the amorphous silicon film is dopedwith a metal element that accelerates crystallization, the amorphoussilicon film is irradiated with laser light (emitted from excimer laserat an energy density of 50 to 150 mJ/cm²) from the back side of thesubstrate, and then heat treatment is performed. Thus formed is acrystalline silicon film in which crystal grains have a uniform grainsize. The metal element used here is nickel. A solution containing 5 ppmof nickel that accelerates crystallization is applied after an oxidefilm is formed on the surface of the amorphous silicon film by using asolution containing ozone. Then, laser light of XeCl laser is irradiatedfrom the back side of the substrate at an energy density of 85 mJ/cm², afrequency of 30 Hz, and a scanning rate of 0.1 mm/sec. in 132 shots.

[0141] Next, heat treatment (at 450° C. for an hour) for dehydrogenationis followed by heat treatment (at 600° C. for twelve hours) forcrystallization. Through this crystallization, a silicon film(polysilicon film) which has a crystal structure of a uniform grain size(about 5 μm) is obtained.

[0142] A gettering step may be interposed here to remove Ni from aregion that serves as an active layer of a TFT. In this case, the regionfor serving as the active layer of a TFT is covered with a mask (siliconoxide film), a part of the crystalline silicon film is doped withphosphorus (P) or argon (Ar), and heat treatment (at 600° C. for twelvehours in a nitrogen atmosphere) is performed.

[0143] Next, unnecessary portions of the silicon film with a crystalstructure are removed by patterning to form a semiconductor layer 404(FIG. 6C). A top view of the pixel after the semiconductor layer 404 isformed is shown in FIG. 6D. A sectional view taken along the dotted lineA-A′ in FIG. 6D corresponds to FIG. 6C.

[0144] In order to a storage capacitor, a mask 405 is formed next andthen a region 406 of the semiconductor layer (a region that serves asthe storage capacitor) is doped with phosphorus (FIG. 7A).

[0145] The mask 405 is removed and an insulating film is formed to coverthe semiconductor layer. Thereafter, a mask 407 is formed and theinsulating film on the region 406 that serves as the storage capacitoris removed (FIG. 7B).

[0146] The mask 407 is then removed and an insulating film (gateinsulating film) 408 a is formed by thermal oxidization. Through thethermal oxidization, the final thickness of the gate insulating filmbecomes 80 nm. An insulating film 408 b formed on the region 406 thatserves as the storage capacitor is thinner than the insulating film onthe other regions (FIG. 7C). A top view of the pixel at this point isshown in FIG. 7D. A sectional view taken along the dotted line B-B′ inFIG. 7D corresponds to FIG. 7C. A region indicated by the dot-dash linein FIG. 7D is the region on which the thin insulating film 408 b isformed.

[0147] The next step is channel doping in which a region that serves asa channel region of a TFT is selectively or entirely doped with a lowconcentration of p-type or n-type impurity element. The channel dopingstep is a step for controlling the threshold voltage of the TFT. Here,boron is doped by ion doping in which diborane (B₂H₆) is subjected toplasma excitation without mass separation. Ion implantation involvingmass separation may be employed instead.

[0148] A mask 409 is formed on the insulating film 408 a and 408 b toform a contact hole that reaches the scanning line 402 (FIG. 8A). Afterthe contact hole is formed, the mask is removed.

[0149] A conductive film is then formed and patterned to form a gateelectrode 410 and a capacitance wiring line 411 (FIG. 5B). Used here isa laminate of a silicon film (150 nm in thickness) doped with phosphorusand a tungsten silicide film (150 nm in thickness). The storagecapacitor is constituted of the capacitance wiring line 411 and theregion 406 of the semiconductor layer with the insulating film 408 b asdielectric.

[0150] Next, using the gate electrode 410 and the capacitance wiringline 411 as masks, the semiconductor layer is doped with a lowconcentration of phosphorus in a self-aligning manner (FIG. 8C). A topview of the pixel at this point is shown in FIG. 5D. A sectional viewtaken along the dotted line C-C′ in FIG. 8D corresponds to FIG. 8C. Theconcentration of the phosphorus concentration is adjusted so as to reach1×10¹⁶ to 5×10¹⁸ atoms/cm³, typically, 3×10¹⁷ to 3×10¹⁸ atoms/cm³.

[0151] A mask 412 is formed and the semiconductor layer is doped with ahigh concentration of phosphorus to form a high concentration impurityregion 413 that serves as a source region or a drain region (FIG. 9A).The concentration of the phosphorus in the high concentration impurityregion is adjusted so as to reach 1×10²⁰ to 1×10²¹ atoms/cm³ (typically,2×10²⁰ to 5×10²⁰ atoms/cm³). Of the semiconductor layer 404, a regionthat overlaps the gate electrode 410 serves as a channel formationregion 414 and a region covered with the mask 412 forms a lowconcentration impurity region 415 to function as an LDD region. Afterdoping of the impurity element is finished, the mask 412 is removed.

[0152] Though not shown in the drawings, a region for forming ann-channel TFT is covered with a mask and the semiconductor layer isdoped with boron to form a source region or drain region for a p-channelTFT of a driving circuit formed on the same substrate on which pixelsare formed.

[0153] After the mask 412 is removed, a passivation film 416 is formedto cover the gate electrode 410 and the capacitance wiring line 411. Thepassivation film prevents oxidization of the gate electrode andfunctions as an etching stopper in a later leveling step. A siliconoxide film with a thickness of 70 nm is used here for the passivationfilm. The next step is heat treatment for activating the n-type andp-type impurity elements used to dope the semiconductor layer indifferent concentrations. Heat treatment here is conducted at 950° C.for 30 minutes.

[0154] An interlayer insulating film 417 is formed next from an organicresin material or a silicon material. A silicon oxynitride film with athickness of 1 μm is used here, and is leveled by etch back. Contactholes to reach the semiconductor layer are formed to form an electrode418 and a source wiring line 419. In Embodiment 3, the electrode 418 andthe source wiring line 419 are a laminate of three layers that areformed in succession by sputtering. The three layers are a Ti film witha thickness of 100 nm. an aluminum film containing Ti with a thicknessof 300 nm, and a Ti film with a thickness of 150 nm (FIG. 9B). Asectional view taken along the dotted line D-D′ in FIG. 9C correspondsto FIG. 9B.

[0155] After hydrogenation treatment, a laminate of a silicon oxynitridefilm (500 nm in thickness) and a BCB film (1 μm in thickness) is formedas an interlayer insulating film 420 (FIG. 10A). A conductive film (100nm in thickness) which is capable of shielding against light is formedon the interlayer insulating film 420, and patterned to form alight-shielding layer 421. A silicon oxynitride film with a thickness of150 nm is formed next as an interlayer insulating film 422. Then, acontact hole to reach the electrode 418 is formed. A transparentconductive film (here, an indium tin oxide, ITO, film) with a thicknessof 100 nm is formed and then patterned to form pixel electrodes 423 and424. A sectional view taken along the dotted line E-E′ in FIG. 10Bcorresponds to FIG. 10A.

[0156] In this way, a pixel TFT that is an n-channel TFT and a storagecapacitor with enough capacitance (51.5 fF) are formed in the pixelportion while a sufficient area (aperture ratio: 76.5%) is secured for adisplay region (pixel size: 26 μm×26 μm).

[0157] This embodiment is merely an example and the present invention isnot limited to the process of this embodiment. For example, theconductive films in this embodiment may he formed of an element selectedfrom the group consisting of tantalum (Ta), titanium (Ti), molybdenum(Mo), tungsten (W), chromium (Cr), and silicon (Si), or of an alloycontaining a combination of the elements listed above (typically a Mo-Walloy or a Mo-Ta alloy). A silicon oxide film, a silicon nitride film, asilicon oxynitride film, or an organic resin material (such aspolyimide, acrylic, polyamide, polyimideamide, BCB (benzocyclobutene))film can be used for the insulating films of this embodiment.

[0158] TFTs obtained in accordance with Embodiment 3 exhibit excellentelectric characteristics.

[0159] Fluctuation in characteristics between the TFTs is small. Inparticular, fluctuation in OFF current value (L/W=8 μm/8 μm) betweenTFTs in this embodiment is smaller than the fluctuation of thecomparative sample, as shown in FIGS. 11A and 11B. The comparativesample in FIGS. 11A and 11B is TFTs obtained under the same conditionsas the TFTs of this embodiment except that only the laser lightirradiation from the back side is omitted in manufacturing the TFTs ofthe comparative sample. FIG. 11A shows the OFF current value in 1V ofthe difference in voltage between a source region and a drain region ofa TFT. FIG. 11B shows the OFF current value in 5V of the difference involtage between a source region and a drain region of a TFT. As shown inFIG. 12, fluctuation in field effect mobility between TFTs of thisembodiment is also small.

[0160] As shown in FIGS. 13A and 13B and FIG. 14, fluctuation incharacteristics between TFTs with a channel size that satisfies L/W=50μm/50 μm is even smaller. FIG. 13A shows the OFF current value in 1V ofthe difference in voltage between a source region and a drain region ofa TFT. FIG. 13B shows the OFF current value in 5V of the difference involtage between a source region and a drain region of a TFT is 5 V.

[0161] In Embodiment 3, a transparent conductive film is used for apixel electrode to manufacture an active matrix substrate for atransmissive display device. However, it is also possible to manufacturean active matrix substrate for a reflective display device if a pixelelectrode is formed from a reflective material.

[0162] [Embodiment 4]

[0163] The description in Embodiment 3 takes as an example a top gateTFT. The present invention is also applicable to a bottom gate TFT shownin FIGS. 16A and 16B.

[0164]FIG. 16A is an enlarged top view of one of pixels in a pixelportion. A section taken along the dotted line A-A′ in FIG. 16Acorresponds to the sectional structure of the pixel portion in FIG. 16B.

[0165] In the pixel portion shown in FIGS. 16A and 16B, n-channel TFTsconstitute the pixel TFT portion. A gate electrode 52 is formed on asubstrate 51. A first insulating film 53 a is formed on the gateelectrode from silicon nitride and a second insulating film 53 b isformed on the first insulating film from silicon oxide. An active layeris formed on the second insulating film. The active layer is composed ofsource and drain regions 54 to 56, channel formation regions 57 and 58,and LDD regions 59 and 60. Each LDD region is placed between one channelformation region and one source or drain region. The channel formationregions 57 and 58 are protected by insulating layers 61 and 62,respectively. Contact holes are formed in a first interlayer insulatingfilm 63 that covers the insulating layers 61 and 62 and the activelayer. A wiring line 64 is formed to be connected to the source region54 and a wiring line 65 is formed to be connected to the drain region56. A passivation film 66 is formed on the wiring lines. A secondinterlayer insulating film 67 is formed on the passivation film. A thirdinterlayer insulating film 68 is formed on the second interlayerinsulating film. A pixel electrode 69 is formed from a transparentconductive film such as an ITO film or a SnO₂ film and is connected tothe wiring line 65. Denoted by 70 is a pixel electrode adjacent to thepixel electrode 69.

[0166] In this embodiment, the active layer is formed in accordance withEmbodiment Mode. First, the gate electrode 52 is formed on the substrate51. The first insulating film 53 a is formed on the gate electrode fromsilicon nitride and the second insulating film 53 b is formed on thefirst insulating film from silicon oxide. Then, an amorphous siliconfilm is formed. The amorphous silicon film is doped with nickel byapplying an aqueous solution that contains nickel, or by forming a verythin nickel film through sputtering. Next, the amorphous silicon film isirradiated with laser light (an energy density of 50 to 150 mJ/cm²) fromthe back side of the substrate and then subjected to heat treatment toform a crystalline silicon film. The crystalline silicon film thus canhave crystal masses with an uniform size. According to the presentinvention, a crystalline silicon film which is uniform over the entiresurface can be obtained irrespective of the presence or absence of agate electrode under the insulating film that is under the crystallinesilicon film. Nickel is then removed or reduced by gettering. Thecrystalline silicon film is patterned to form the active layer.

[0167] Although a bottom gate TFT of channel stop type is described asan example in this embodiment, the present invention is not particularlylimited thereto.

[0168] The gate wiring line of the pixel TFT in the pixel portion ofthis embodiment is arranged so as to form a double gate structure.However, the present invention may take a triple gate structure or othermulti-gate structure in order to reduce fluctuation in OFF current. Asingle gate structure may also be employed in order to improve theaperture ratio.

[0169] A capacitor portion of the pixel portion is composed of acapacitance wiring line 71 and the drain region 56 with the firstinsulating film and the second insulating film as dielectric.

[0170] The pixel portion shown in FIGS. 16A and 16B is merely an exampleand the present invention is not particularly limited to the abovestructure.

[0171] [Embodiment 5]

[0172] Embodiment 5 describes a process of manufacturing an activematrix liquid crystal display device from the active matrix substratefabricated in Embodiment 3. The description is given with reference toFIG. 17.

[0173] After the active matrix substrate as illustrated in FIG. 10 isobtained in accordance with Embodiment 3, an orientation film is formedon the active matrix substrate of FIG. 10 and subjected to rubbingtreatment. In this embodiment, an organic resin film such as an acrylicresin film is patterned to form columnar spacers in desired positions inorder to keep the distance between the substrates before the orientationfilm is formed. The columnar spacers may be replaced by sphericalspacers sprayed onto the entire surface of the substrate.

[0174] An opposite substrate is prepared next. The opposite substratehas a color filter in which colored layers and light-shielding layersare arranged with respect to the respective pixels. A light-shieldinglayer is also placed in the driving circuit portion. A leveled film isformed to cover the color filter and the light-shielding layer. On theleveled film, an opposite electrode is formed from a transparentconductive film in the pixel portion. An orientation film is formed overthe entire surface of the opposite substrate and is subjected to rubbingtreatment.

[0175] Then the opposite substrate is bonded to the active matrixsubstrate on which the pixel portion and the driving circuits areformed, through a sealing member. The sealing member has filler mixedtherein and the filler, together with the columnar spacers, keeps thedistance between the two substrates while they are bonded. Thereafter aliquid crystal material is injected between the substrates and anencapsulant (not shown) is used to completely seal the substrates. Aknown liquid crystal material can be used. The active matrix liquidcrystal display device is thus completed. If necessary, the activematrix substrate or the opposite substrate is cut into pieces withdesired shapes. The display device may be appropriately provided with apolarizing plate using a known technique. Then FPCs are attached using aknown technique.

[0176] The structure of the thus obtained liquid crystal module isdescribed with reference to the top view in FIG. 17.

[0177] A pixel portion 804 is placed in the center of an active matrixsubstrate 801. In FIG. 17, a source signal line driving circuit 802 fordriving source signal lines is positioned above the pixel portion 804.Gate signal line driving circuits 803 for driving gate signal lines areplaced at the left and right sides of the pixel portion 804. Althoughthe gate signal line driving circuits 803 are symmetrical with respectto the pixel portion in this embodiment, the liquid crystal module mayhave only one gate signal line driving circuit at one side of the pixelportion. A designer can choose the arrangement that suits betterconsidering the substrate size of the liquid crystal module, or thelike. However, the symmetrical arrangement of the gate signal linedriving circuits shown in FIG. 17 is preferred in terms of circuitoperation reliability, driving efficiency, and the like.

[0178] Signals are inputted to the driving circuits from flexibleprinted circuits (FPC) 805. The FPCs 805 are press-fit through ananisotropic conductive film or the like after opening contact holes inthe interlayer insulating film and resin film and forming a connectionelectrode so as to reach the wiring lines arranged in given places ofthe substrate 801. The connection electrode is formed from ITO in thisembodiment.

[0179] A sealing agent 807 is applied to the substrate to surround thedriving circuits and the pixel portion. An opposite substrate 806 isbonded to the substrate 801 through the sealing agent 807 while a spacerformed in advance on the active matrix substrate keeps the gap betweenthe two substrates (the substrate 801 and the opposite substrate 806)constant. A liquid crystal element is injected through an area of thesubstrate that is not coated with the sealing agent 807. The substratesare then sealed by an encapsulant 808. The liquid crystal module iscompleted through the above steps.

[0180] Although all of the driving circuits are formed on the substratehere, several ICs may be used for some of the driving circuits.

[0181] In addition, this embodiment can be applied to the active matrixsubstrate obtained in Embodiment 4 substituted for that obtained inEmbodiment 3.

[0182] [Embodiment 6]

[0183] This embodiment shows an example of manufacturing a lightemitting display device which has an EL (electro luminescence) element.

[0184] A pixel portion, a source side driving circuit, and a gate sidedriving circuit are formed on a substrate with an insulating surface(for example, a glass substrate, a crystallized glass substrate, aplastic substrate, and the like). The pixel portion and the drivingcircuits can be obtained in accordance with the description inEmbodiment 1 or Embodiment 2. When a semiconductor film with a crystalstructure is formed for active layers of TFTs in the pixel portion anddriving circuits, an amorphous semiconductor film is doped with a metalelement that accelerates crystallization and then irradiated with laserlight with energy which is not large enough to melt the semiconductorfilm. As a result. the metal element is diffused in the solidsemiconductor film, which helps crystallization in later heat treatment.

[0185] When the semiconductor film is crystallized without using a metalelement that accelerates crystallization, it also improves latercrystallization to irradiate laser light with energy which is not largeenough to melt a semiconductor film before crystallization. Impurities(elements which has high diffusion constant in the semiconductor film,hydrogen, for example) contained in the semiconductor film with anamorphous structure are diffused in the solid semiconductor film andcrystallization in later heat treatment is performed better. When asemiconductor film with a uniform amorphous structure is crystallized byirradiation of laser light with energy which is not large enough to meltthe semiconductor film, a semiconductor film with a uniform crystalstructure can be obtained. Accordingly, TFTs which have as their activelayers this semiconductor film with a crystal structure are uniform incharacteristics to reduce fluctuation in luminance.

[0186] The pixel portion and the driving circuits are covered with aseal member, which in turn is covered with a protective film. Further,sealing is completed by a cover member with an adhesive. The covermember is desirably formed from the same material as the substrate, forexample, a glass substrate, in order to prevent deformation due to heatand external force. The cover member is processed by sand blasting orthe like to have a concave shape (depth: 3 to 10 μm). Desirably, thecover member is further processed to have a dent (depth: 50 to 200 μm)in which a drying agent is placed. If more than one EL modules are to beobtained from one sheet, CO₂ laser or the like is used to cut out amodule with its ends flush after bonding the cover member to thesubstrate.

[0187] Described next is the sectional structure of the device. Aninsulating film is formed on a substrate. A pixel portion and a gateside driving circuit are formed on the insulating film. The pixelportion is composed of a plurality of pixels each including a currentcontrolling TFT and a pixel electrode that is electrically connected toa drain of the current controlling TFT. The gate side driving circuit isbuilt from a CMOS circuit in which an n-channel TFT and a p-channel TFTare combined. These TFTs can be manufactured in accordance withEmbodiment 1 or Embodiment 2.

[0188] The pixel electrode functions as an anode of an EL element. Abank is formed on each end of the pixel electrode. An EL layer andcathode of the EL element are formed on the pixel electrode.

[0189] The EL layer (a layer which emits light and in which carriersmoves for light emission) is a combination of a light emitting layer, anelectric charge transporting layer, and an electric charge injectinglayer. For example, a low molecular weight organic EL material or a highmolecular weight organic EL material is used for the EL layer. A thinfilm of a light emitting material (singlet compound) that emits lightfrom singlet excitation (fluorescence), or a thin film of a lightemitting material (triplet compound) that emits light from tripletexcitation (phosphorescence) may be used in the EL layer. An inorganicmaterial such as silicon carbide may be used for the electric chargetransporting layers and electric charge injecting layers. These organicEL materials and inorganic materials can be known materials.

[0190] The cathode also functions as a common wiring line to all thepixels and is electrically connected to an FPC through a connectionwiring line. All the elements included in the pixel portion and gateside driving circuit are covered with the cathode, the seal member, andthe protective film.

[0191] The seal member is preferably formed from a transparent ortranslucent material with respect to visible light. It is also desirableto use for the seal member a material that transmits as little moistureand oxygen as possible.

[0192] After the light emitting element is completely covered with theseal member, the protective film formed of a DLC film or the like isplaced at least on the surface (the exposed surface) of the seal member.The protective film may cover all the surfaces including the back sideof the substrate. However, it is important to avoid forming theprotective film in a portion where an external input terminal (FPC) isto be placed. In order to avoid forming the protective film in thisportion. a mask may be used or the external input terminal portion maybe covered with a tape such as Teflon (registered trade mark) that isused as a masking tape in CVD apparatus.

[0193] By sealing as above using the seal member and the protectivefilm, the EL element is completely shut off from the outside andexternal substances such as moisture and oxygen, that acceleratesdegradation by oxidization of EL layer, are prevented from entering theelement. Accordingly, a light emitting device with high reliability canbe obtained.

[0194] The light emitting device can emit light in the reverse directionto the above structure if the pixel electrode serves as a cathode and anEL layer and an anode are layered on the cathode.

[0195] The present invention can reduce fluctuation in ON current(I_(on)) of TFTs (TFTs for supplying current to a driving circuit or toan OLED in a pixel) arranged so that a constant current flows in a pixelelectrode, and therefore can reduce fluctuation in luminance.

[0196] [Embodiment 7]

[0197] The TFT fabricated by implementing the present invention can beutilized for various modules (active matrix liquid crystal module,active matrix EL module and active matrix EC module). Namely, all of theelectronic apparatuses are completed by implementing the presentinvention.

[0198] Following can be given as such electronic apparatuses: videocameras; digital cameras; head mounted displays (goggle type displays);car navigation systems; projectors; car stereo; personal computers;portable information terminals (mobile computers, mobile phones orelectronic books etc.) etc. Examples of these are shown in FIGS. 18A to18F, 19A to 19D and 20A to 20C.

[0199]FIG. 18A is a personal computer which comprises: a main body 2001;an image input section 2002; a display section 2003; and a keyboard2004.

[0200]FIG. 18B is a video camera which comprises: a main body 2101; adisplay section 2102; a voice input section 2103; operation switches2104; a battery 2105 and an image receiving section 2106.

[0201]FIG. 18C is a mobile computer which comprises: a main body 2201; acamera section 2202; an image receiving section 2203; operation switches2204 and a display section 2205.

[0202]FIG. 18D is a goggle type display which comprises: a main body2301; a display section 2302; and an arm section 2303.

[0203]FIG. 18E is a player using a recording medium which records aprogram (hereinafter referred to as a recording medium) which comprises:a main body 2401; a display section 2402; a speaker section 2403; arecording medium 2404; and operation switches 2405. This apparatus usesDVD (digital versatile disc), CD, etc. for the recording medium, and canperform music appreciation, film appreciation, games and use forInternet.

[0204]FIG. 18F is a digital camera which comprises: a main body 2501; adisplay section 2502; a view finder 2503; operation switches 2504; andan image receiving section (not shown in the figure).

[0205]FIG. 19A is a front type projector which comprises: a projectionsystem 2601; and a screen 2602. The present invention can be applied tothe liquid crystal module 2808 forming a part of the projection system2601.

[0206]FIG. 19B is a rear type projector which comprises: a main body2701; a projection system 2702; a mirror 2703; and a screen 2704. Thepresent invention can be applied to the liquid crystal module 2808forming a part of the projection system 2702.

[0207]FIG. 19C is a diagram which shows an example of the structure of aprojection system 2601 and 2702 in FIGS. 19A and 19B, respectively. Eachof projection systems 2601 and 2702 comprises: an optical light sourcesystem 2801; mirrors 2802 and 2804 to 2806; a dichroic mirror 2803; aprism 2807; a liquid crystal module 2808; a phase differentiating plate2809; and a projection optical system 2810. The projection opticalsystem 2810 comprises an optical system having a projection lens. Thoughthis embodiment shows an example of 3-plate type, this is not to limitto this embodiment and a single plate type may be used for instance.Further, an operator may appropriately dispose an optical lens, a filmwhich has a function to polarize light, a film which adjusts a phasedifference or an IR film, etc. in the optical path shown by an arrow inFIG. 19C.

[0208]FIG. 19D is a diagram showing an example of a structure of anoptical light source system 2801 in FIG. 19C. In this embodiment, theoptical light source system 2801 comprises: a reflector 2811; a lightsource 2812; lens arrays 2813 and 2814; a polarizer conversion element2815; and a collimator lens 2816. Note that the optical light sourcesystem shown in FIG. 19D is merely an example and the structure is notlimited to this example. For instance, an operator may appropriatelydispose an optical lens. a film which has a function to polarize light,a film which adjusts a phase difference or an IR film, etc.

[0209] Note that the projectors shown FIGS. 19A to 19D are the cases ofusing a transmission type electro-optical device, and applicableexamples of a reflection type electro-optical device and an EL moduleare not shown.

[0210]FIG. 20A is a portable telephone which comprises: a main body2901: a voice output section 2902; a voice input section 2903; a displaysection 2904: operation switches 2905; all antenna 2906; and an imageinput section (CCD, image sensor, etc.) 2907 etc.

[0211]FIG. 20B is a portable book (electronic book) which comprises: amain body 3001; display sections 3002 and 3003; a recording medium 3004;operation switches 3005 and an antenna 3006 etc.

[0212]FIG. 20C is a display which comprises: a main body 3101; asupporting section 3102; and a display section 3103 etc. In addition,the display shown in FIG. 20C is small and medium type or large type,for example, screen of the display sized 5 to 20 inches. Moreover, it ispreferable to mass-produce by executing a multiple pattern using asubstrate sized 1×1m to form such sized display section.

[0213] As described above, the applicable range of the present inventionis very large, and the invention can be applied to electronicapparatuses of various areas. Note that the electronic devices of thisembodiment can be achieved by utilizing any combination of constitutionsin Embodiments 1 to 6.

[0214] [Embodiment 8]

[0215] Embodiment 2 shows an example of obtaining a semiconductor filmwith a crystal structure of uniform crystal grains by doping of a metalelement that accelerates crystallization, irradiation of laser light(pulse oscillation excimer laser light) with energy which is not largeenough to melt a semiconductor film, and heat treatment. This embodimentshows an example in which a continuous wave laser is employed.

[0216] A nickel thin film is formed on a semiconductor film with anamorphous structure in accordance with Embodiment 2. Thereafter, thesemiconductor film with an amorphous structure, on which the nickel thinfilm is formed, is irradiated with laser light (continuous wave laserlight) with an energy which is not large enough to melt thesemiconductor film.

[0217] First, laser light emitted from a 10 W power or 6 W power ofcontinuous wave YVO₄ laser is converted into harmonic (second harmonicto fourth harmonic) by a non-linear optical element. Alternatively, YVO₄crystals and a non-linear optical element are put in a resonator to emitharmonic. Then, the harmonic is preferably shaped by an optical systemto form a rectangle or an ellipse on the irradiated surface beforeirradiating the semiconductor film with an amorphous structure.

[0218] It is important for the laser light to have an energy which isnot large enough to melt the semiconductor film. The energy density isset to 0.01 to 100 MW/cm² (preferably 0.1 to 10 MW/cm²) and the scanningrate is set to 0.5 to 2000 cm/sec., preferably higher than 20 cm/sec.,to make the metal element in the film diffuse uniformly throughout thesolid semiconductor film to increase the number of crystal nuclei.

[0219] Next, the semiconductor film is crystallized by heat treatment toform a semiconductor film with a crystal structure. Here, heat treatmentat 450° C. for an hour is followed by heat treatment at 600° C. fortwelve hours. The thus obtained semiconductor film with a crystalstructure has a uniform grain size throughout the film.

[0220] The obtained semiconductor film with a crystal structure is thenpatterned to form a semiconductor layer. After washing the surface ofthe semiconductor layer with an etchant containing fluoric acid, aninsulating film mainly containing silicon is formed to serve as a gateinsulating film. The surface washing and formation of the gateinsulating film are desirably carried out in succession without exposingthe substrate to the air. Then, the surface of the gate insulating filmis washed and a gate electrode is formed. The semiconductor layer isdoped with an impurity element that gives a semiconductor the n-typeconductivity (such as P or As), here, phosphorus, to form a sourceregion 209 and a drain region 210. After the doping, the impurityelement is activated by heat treatment, irradiation of intense light, orirradiation of laser light. The impurity element is activated, and atthe same time, plasma damage to the gate insulating film and plasmadamage to the interface between the gate insulating film and thesemiconductor layer can be repaired. When heat treatment is employed asactivation measures, activation and gettering can be achievedsimultaneously. Gettering here utilizes phosphorus doped in the sourceregion or drain region. The metal element that accelerates crystalgrowth, doped before crystallization, is desirably removed from orreduced in the crystalline semiconductor film through gettering afterthe crystallization.

[0221] The subsequent steps include forming an interlayer insulatingfilm, hydrogenating the semiconductor layer, forming contact holes thatreach the source region and the drain region, and forming a sourceelectrode and a drain electrode. A TFT is thus completed.

[0222] Although the thus obtained TFT has a plurality of grainboundaries in a channel formation region, it has highly uniform crystalmasses. There is little fluctuation between TFTs formed on the samesubstrate.

[0223] This embodiment can be combined with any one of Embodiments 1through 7.

[0224] [Embodiment 9]

[0225] Embodiment 2 shows an example of obtaining a semiconductor filmwith a crystal structure of uniform crystal grains by doping of a metalelement that accelerates crystallization, irradiation of laser lightwith energy which is not large enough to melt a semiconductor film, andheat treatment. This embodiment shows an example in which asemiconductor film is crystallized without using a metal element thataccelerates crystallization.

[0226] A semiconductor film with an amorphous structure is formed on abase insulating film in accordance with Embodiment 2.

[0227] Next, the semiconductor film is irradiated with laser light withenergy which is not large enough to melt the semiconductor film from thefront side or back side. The laser light may be light emitted from apulse oscillation laser or from a continuous wave laser. Examples oflaser light that can be employed include light emitted from one or morekinds selected from the group consisting of a pulse oscillation excimerlaser, a pulse oscillation Ar laser, a pulse oscillation Kr laser, acontinuous wave excimer laser, a continuous wave Ar laser, and acontinuous wave Kr laser, or one or more kinds selected from the groupconsisting of a continuous wave YAG laser, a continuous wave YVO₄ laser,a continuous wave YLF laser, a continuous wave YAG laser, a continuouswave glass laser, a continuous wave ruby laser, a continuous wavealexandrite laser, a continuous wave Ti: sapphire laser, a pulseoscillation YAG laser, a pulse oscillation YVO₄ laser, a pulseoscillation YLF laser, a pulse oscillation YAlO₃ laser, a pulseoscillation glass laser, a pulse oscillation ruby laser, a pulseoscillation alexandrite laser, and a pulse oscillation Ti: sapphirelaser.

[0228] The energy of laser light in this embodiment is not large enoughto melt an amorphous silicon film, is not large enough to change thesurface state of the film, and is large enough to allow impurities(typically hydrogen) to move in the solid semiconductor film. When apulse oscillation laser is employed, the energy density is set to 50 to150 mJ/cm².

[0229] As has been described, irradiation of laser light with enery,which is not large enough to melt a semiconductor film with an amorphousstructure before crystallization makes impurities included in thesemiconductor film (elements with a high diffusion constant or a highdegree of solid solution in the semiconductor film, hydrogen, forexample) diffuse in the solid film for improved crystallization later.Hydrogen has higher diffusion constant than nickel and can be diffusedin the film by laser light with relatively low energy.

[0230] The semiconductor film with an amorphous structure iscrystallized next. The film may be crystallized by irradiation of laserlight that has an energy large enough to melt the semiconductor film, orby heat treatment using a furnace, or by irradiation of intense lightemitted from a lamp.

[0231] When laser light with an energy which is large enough to melt thesemiconductor film is employed as measures for crystallization, laserirradiation is conducted twice in total (once at an energy level whichis not high enough to melt the semiconductor film and once at an energylevel which is high enough to melt the semiconductor film). This ispreferable because uniform crystallization can be achieved in a shortperiod of time.

[0232] If excimer laser light is used as crystallization measures,irradiation of pulse oscillation laser light with an energy density of50 to 150 mJ/cm² is immediately followed by irradiation of pulseoscillation laser light with an energy density of 200 mJ/cm² or more forcrystallization. If YVO₄ laser light is used, irradiation of continuouswave laser light with energy which is not large enough to melt thesemiconductor film at a higher scanning rate than 200 cm/sec. isimmediately followed by irradiation of continuous wave laser light withenergy which is large enough to melt the semiconductor film at a reducedscanning rate, 100 cm/sec. or lower, for crystallization.

[0233] In this way, when a semiconductor film with a uniform amorphousstructure obtained by irradiation of laser light with energy which isnot large enough to melt the semiconductor film is crystallized toobtain a semiconductor film with a uniform crystal structure.Accordingly, TFTs which have as their active layers this semiconductorfilm with a crystal structure are uniform in characteristics to reduceuneven display and fluctuation in luminance.

[0234] Although a metal element that accelerates crystallization is notused in the example shown here, irradiation of laser light with energywhich is not large enough to melt a semiconductor film may precededoping of a metal element that accelerates crystallization.

[0235] This embodiment can be combined with any one of Embodiments 1through 8.

[0236] According to the present invention, a TFT in which crystal masseshave a uniform size and a channel formation region has a plurality ofcrystal masses can be obtained and a semiconductor device with verylittle fluctuation is provided. In particular, uneven display due tofluctuation in TFT characteristics can be reduced in a liquid crystaldisplay device. Moreover, the present invention can reduce fluctuationin ON current (I_(on)) of TFTs arranged for flowing a constant current(TFTs for supplying a current to a driving circuit or to an OLED in apixel) in a semiconductor device which has an OLED, and therefore canreduce fluctuation in luminance.

[0237] The present invention is also capable of crystallizing asemiconductor film at a lower temperature for heat treatment in ashorter period of time than in prior art.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising: a first step of forming a first semiconductor filmcomprising an amorphous structure on a substrate with an insulatingsurface; a second step of doping the first semiconductor film with ametal element; a third step of irradiating light to the firstsemiconductor film from the back side of the substrate; and a fourthstep of forming a second semiconductor film comprising a crystalstructure by heating the first semiconductor film.
 2. A method ofmanufacturing a semiconductor device, comprising: a first step offorming a first semiconductor film comprising an amorphous structure ona substrate with an uneven insulating surface; a second step of dopingthe first semiconductor film with a metal element; a third step ofirradiating light to the first semiconductor film from the back side ofthe substrate; and a fourth step of forming a second semiconductor filmcomprising a crystal structure by heating the first semiconductor film.3. A method of manufacturing a semiconductor device according to claim1, wherein crystal masses of in the second semiconductor aresubstantially uniform in size.
 4. A method of manufacturing asemiconductor device according to claim 2, wherein crystal masses of inthe second semiconductor are substantially uniform in size.
 5. A methodof manufacturing a semiconductor device according to claim 1, whereinsizes of crystal masses in the second semiconductor film areapproximately 1 to 20 μm.
 6. A method of manufacturing a semiconductordevice according to claim 2, wherein sizes of crystal masses in thesecond semiconductor film are approximately 1 to 20 μm.
 7. A method ofmanufacturing a semiconductor device according to claim 1, wherein thelight is pulse oscillation laser light with an energy density of 50 to150 mJ/cm².
 8. A method of manufacturing a semiconductor deviceaccording to claim 2, wherein the light is pulse oscillation laser lightwith an energy density of 50 to 150 mJ/cm².
 9. A method of manufacturinga semiconductor device according to claim 1, wherein the light isemitted from one or more kinds selected from the group consisting of apulse oscillation excimer laser, a pulse oscillation Ar laser, a pulseoscillation Kr laser, a continuous wave excimer laser, a continuous waveAr laser, and a continuous wave Kr laser.
 10. A method of manufacturinga semiconductor device according to claim 2, wherein the light isemitted from one or more kinds selected from the group consisting of apulse oscillation excimer laser, a pulse oscillation Ar laser, a pulseoscillation Kr laser, a continuous wave excimer laser, a continuous waveAr laser, and a continuous wave Kr laser.
 11. A method of manufacturinga semiconductor device according to claim 1, wherein the light isemitted from one or more kinds selected from the group consisting of acontinuous wave YAG laser, a continuous wave YVO₄ laser, a continuouswave YLF laser, a continuous wave YAlO₃ laser, a continuous wave glasslaser, a continuous wave ruby laser, a continuous wave alexandritelaser, a continuous wave Ti : sapphire laser, a pulse oscillation YAGlaser, a pulse oscillation YVO₄ laser, a pulse oscillation YLF laser, apulse oscillation YAlO₃ laser, a pulse oscillation glass laser, a pulseoscillation ruby laser, a pulse oscillation alexandrite laser, and apulse oscillation Ti : sapphire laser.
 12. A method of manufacturing asemiconductor device according to claim 2, wherein the light is emittedfrom one or more kinds selected from the group consisting of acontinuous wave YAG laser, a continuous wave YVO₄ laser, a continuouswave YLF laser, a continuous wave YAlO, laser, a continuous wave glasslaser, a continuous wave ruby laser, a continuous wave alexandritelaser, a continuous wave Ti: sapphire laser, a pulse oscillation YAGlaser, a pulse oscillation YVO₄ laser, a pulse oscillation YLF laser, apulse oscillation YAlO₃ laser, a pulse oscillation glass laser, a pulseoscillation ruby laser. a pulse oscillation alexandrite laser, and apulse oscillation Ti : sapphire laser.
 13. A method of manufacturing asemiconductor device according to claim 1, wherein the metal element isone that accelerates crystallization.
 14. A method of manufacturing asemiconductor device according to claim 2, wherein the metal element isone that accelerates crystallization.
 15. A method of manufacturing asemiconductor device according to claim 1, wherein the energy of thelight is not large enough to melt the semiconductor film.
 16. A methodof manufacturing a semiconductor device according to claim 2, whereinthe energy of the light is not large enough to melt the semiconductorfilm.
 17. A method of manufacturing a semiconductor device, comprising:a first step of forming a first semiconductor film comprising anamorphous structure on a substrate with an insulating surface; a secondstep of irradiating the front side or back side of the secondsemiconductor film with light comprising energy which is not largeenough to melt the semiconductor film; and a third step of forming asecond semiconductor film comprising a crystal structure bycrystallizing the first semiconductor film.
 18. A method ofmanufacturing a semiconductor device according to claim 17, wherein thethird step is a step of irradiating the first semiconductor film withlaser light, a step of heating the first semiconductor film, or a stepof heating the first semiconductor film after doping the firstsemiconductor film with a metal element that acceleratescrystallization.
 19. A method of manufacturing a semiconductor deviceaccording to claim
 18. wherein the metal element is one or more kindsselected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir,Pt, Cu, and Au.
 20. A method of manufacturing a semiconductor deviceaccording to claim 17, wherein the light is emitted from one or morekinds selected from the group consisting of a pulse oscillation excimerlaser, a pulse oscillation Ar laser, a pulse oscillation Kr laser, acontinuous wave excimer laser, a continuous wave Ar laser, and acontinuous wave Kr laser.
 21. A method of manufacturing a semiconductordevice according to claim 17, wherein the light is emitted from one ormore kinds selected from the group consisting of a continuous wave YAGlaser, a continuous wave YVO₄ laser, a continuous wave YLF laser, acontinuous wave YAlO₃ laser, a continuous wave glass laser, a continuouswave ruby laser, a continuous wave alexandrite laser, a continuous waveTi : sapphire laser, a pulse oscillation YAG laser, a pulse oscillationYVO₄ laser, a pulse oscillation YLF laser, a pulse oscillation YAlO₃laser, a pulse oscillation glass laser, a pulse oscillation ruby laser,a pulse oscillation alexandrite laser, and a pulse oscillation Ti :sapphire laser.
 22. A method of manufacturing a semiconductor devicecomprising a pixel portion and a driving circuit on the same substrate,comprising: a first step of forming a first semiconductor filmcomprising an amorphous structure on a substrate with an insulatingsurface; a second step of selectively irradiating only a region of thefirst semiconductor film that serves as the pixel portion with lightcomprising energy which is not large enough to melt the semiconductorfilm; and a third step of forming a second semiconductor film comprisinga crystal structure by heating the first semiconductor film.
 23. Asemiconductor device comprising a first region comprising a first TFTand a second region comprising a second TFT on the same substrate,wherein the first TFT comprises as its active layer a semiconductor filmwith a crystal structure. sizes of a plurality of crystal grains arelarger in the first TFT than in the second TFT, and the sizes are lessfluctuated in the first region than in the second region.
 24. Asemiconductor device according to claim 23, wherein the first region isa pixel portion and the second region is a driving circuit.
 25. Asemiconductor device according to claim 23, wherein the semiconductordevice is a liquid crystal display device.
 26. A semiconductor deviceaccording to claim 23, wherein the semiconductor device is an EL module.27. A semiconductor device according to claim 23, wherein thesemiconductor device is a video camera, a digital camera, a navigationsystem for vehicles, a personal computer, a portable informationterminal. or an electronic game machine.